Peachette, as long as you're in the US DON'T ORDER NJOY CARTS--like Kate said--Puresmoker, esmokeytreats & Janty all sell filled carts for 5.00 a box.
(Also, RINSE OUT THOSE USED CARTS & keep an old coffee mug in the cupboard & throw 'em in there--because once you get used to cart liquid consistency and feel more comfortable with dripping, you'll have plenty of 'spare carts' for use.)
For myself, if a new filled-cart is a little loose in my atomizer, it's a one-time-only. I throw it away when I'm done. If it's a good snug fit, I know to keep it for re-use down the road.
Here's something else I've just start doing (because dripping in carts just don't last as long as prefilled). Buy one of those syringes (like Puresmoker sells for under 2 bucks).
When you fill it up from the "bottom up", it lasts just as long as a pre-fill.
Um...on a related note, here's a goofy incident that just happened to me a week ago. My sister also "vapes" (but doesn't visit any of these boards). When I got a new syringe from Puresmoker last week, I showed it to her & said "you gotta get one of these, it's a lot faster than dripping in those carts" & she said "Doug, now you're SHOOTING UP THIS STUFF? I still prefer to just puff it!"
(I'm not kidding--neither was she)